A way of saying you agree with someone.

Lying flat refers to a defeatist lifestyle in Chin, where people stop working, desiring material acquisition, and tap out of any social life – sometimes for good. By lying flat, young people are participating in a kind of non-cooperation movement, rather than surrendering themselves to China's oppressive work culture where they see little hope of social mobility.

1) A way of stating that they're talking about something you know very well or agree with.
2) A way of stating that they're actually talking about you. (Often used as sarcasm).

Said when talking about something you've experienced.

When somebody sais something about the situation your in and its obvious you be like Yeah tell me bout it or when something is said that is obvious

used as an ironic acknowledgment of one's familiarity with a difficult or unpleasant situation or experience described by someone else.